, enclosed apparatus in which heat is produced, either by burning a fuel such as coal, coke, oil, or natural gas, or by passing current through a resistive electrical conductor. The amount of heat that is produced in a furnace can be controlled. To minimize the heat lost by radiation, some furnaces, particularly those used in industry, are covered with insulation, such as firebrick.

II. Building Furnaces

Homes and many public and commercial buildings are heated by hot-air, hot-water, or steam-heating systems. The fuel is burned in a part of the furnace called the firebox, and the heat rises into a combustion chamber. In a hot-air system, hot air then flows directly into insulated ducts and is distributed throughout the building. The advantage of a hot-air system is that the same duct system used to transport hot air in the winter can also be used to transport cool air produced by an air conditioner in the summer. In a hot-water system, water is heated in a boiler located above the combustion chamber, and pipes then carry the hot water to and from radiators installed in the building. In steam-heating systems, pipes carrying water pass through the combustion chamber, and the water is converted into steam that moves through pipes and into the radiators. See Heating, Ventilating, and Air Conditioning.

III. Industrial Furnaces

Industries use a number of different kinds of furnaces. In the metallurgical industry, furnaces are used to roast and smelt ores. A blast furnace smelts iron ore; other types of furnaces, such as the basic-oxygen furnace and the open-hearth furnace, convert liquid iron into liquid steel. Electric-arc furnaces are used to make high-quality steel (see Electric Arc). The properties of rolled or wrought metals are altered in annealing furnaces, which are also used to toughen sheet glass (see Annealing). Salt-bath furnaces are used to heat steel parts before they are hardened by rapid cooling. The ceramics industry uses a furnace called a kiln to fire products made of clay and to set glazes. Kilns are also used to calcinate limestone and to reduce certain ores.